Wiring board and method of manufacturing wiring board

ABSTRACT

A wiring board and method of forming a wiring board including a first substrate, a second substrate having a smaller mounting area than a mounting area of the first substrate, and a base substrate laminated between the first substrate and the second substrate, such that the first substrate extends beyond an edge of the second substrate. An IVH (Interstitial Via Hole) or through hole penetrates the base substrate and vias are formed in at least one of the first substrate or the second substrate.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser. No. 60/938,597, filed May 17, 2007, the entire content of which is incorporated herein by reference.

FIELD OF TECHNOLOGY

The present invention relates to a wiring board formed by combining at least two boards each having a different size mounting area, and a method of manufacturing such a wiring board.

DESCRIPTION OF RELATED ART

Japanese Unexamined Patent Publication H5-152693 discloses technology to solve insufficient rigidity in a wiring board. The technology relates to a wiring board having a reinforced section formed by making an extended portion of a flexible substrate and folding the extended portion.

Further, technology to provide wiring structures with high flexibility is described, for example, in WO 05/029934. This publication discloses a printed wiring board having a first substrate and a second substrate laminated on the first substrate, where the contour of the second substrate is different from the contour of the first substrate. The entire content of each of H5-152693 and WO 05/029934 is incorporated herein by reference.

SUMMARY OF THE INVENTION

A first aspect of the invention relates to a wiring board including a first substrate, a second substrate having a smaller mounting area than that of the first substrate; and a base substrate laminated between the first substrate and the second substrate such that the first substrate extends beyond an edge of the second substrate. An IVH (Interstitial Via Hole) penetrates the base substrate and vias are formed in at least one of the first substrate or the second substrate.

Another aspect of the invention relates to a method of manufacturing a wiring board including forming a base substrate, forming a first insulation layer on a first surface and of the base substrate and a second insulating layer on a second surface of the substrate opposing the first surface. Also included is forming an IVH (Interstitial Via Hole) that penetrates the base substrate, and forming vias in at least one of the first substrate or second substrate. Also included is cutting the first insulating layer in a first area and cutting the second insulating layer in a second area offset from said first area to form a first substrate laminated to a second substrate with the base layer interposed therebetween, the second substrate having a smaller mounting area than that of the first substrate such that the first substrate extends beyond an edge of the second substrate.

In still another aspect of the invention relates to a wiring board including a first substrate, a second substrate having a smaller mounting area than that of the first substrate; and a base substrate laminated between the first substrate and the second substrate such that the first substrate extends beyond an edge of the second substrate. A through hole penetrates the base substrate and vias are formed in at least one of the first substrate or the second substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1A is a side view illustrating a wiring board according to an embodiment of the present invention.

FIG. 1B is a plan view illustrating a wiring board according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view illustrating a wiring board according to an embodiment of the present invention.

FIG. 3 is a cross-sectional view illustrating a wiring board according to an embodiment of the present invention.

FIG. 4 is a cross-sectional view illustrating a wiring board according to an embodiment of the present invention.

FIG. 5 is a cross-sectional view illustrating a wiring board according to an embodiment of the present invention.

FIG. 6 is a cross-sectional view illustrating a wiring board according to an embodiment of the present invention.

FIG. 7A is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7B is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7C is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7D is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7E is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7F is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7G is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7H is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7I is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7J is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7K is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7L is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7M is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7N is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7O is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7P is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7Q is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7R is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7S is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 7T is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 8 is a cross-sectional view of a wiring board according to an embodiment of the present invention.

FIG. 9A is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 9B is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 9C is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 9D is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 9E is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 10 is a cross-sectional view of a wiring board according to an embodiment of the present invention.

FIG. 11A is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 11B is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 12 is a cross-sectional view of a wiring board according to an embodiment of the present invention.

FIG. 13 is a cross-sectional view of a wiring board according to an embodiment of the present invention.

FIG. 14 is a cross-sectional view of a wiring board according to an embodiment of the present invention.

FIG. 15 is a cross-sectional view of a wiring board according to an embodiment of the present invention.

FIG. 16 is a cross-sectional view of a wiring board according to an embodiment of the present invention.

FIG. 17 is a cross-sectional view of a wiring board according to an embodiment of the present invention.

FIG. 18 is a cross-sectional view of a wiring board according to an embodiment of the present invention.

FIG. 19A is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 19B is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

FIG. 19C is an illustration to describe a manufacturing step of a wiring board according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, an embodiment of a wiring board according to a specific example of the present invention is described with reference to the drawings.

As shown in FIG. 1A, wiring board 19 according to a specific example of the present invention has a different thickness on one edge from that on the other edge. The number of layers in the section having a different thickness (thicker section) differs from the number of layers in the thinner section. Namely, wiring board 19 has thick multi-layer section 13 and relatively thin fewer-layer section 14. Multi-layer section 13 is formed by laminating two layers; first substrate 1 and second substrate 2. Fewer-layer section 14 has first substrate 1 which is extended from multi-layer section 13. Thus, as used herein, the term, “multi-layer section” means 2 or more layers or boards, while the term “fewer layer section” means one or more layers or boards.

As shown in FIGS. 1A and 1B, first substrate 1 and second substrate 2 have the same width and different lengths, and one end of first substrate 1 and one end of second substrate 2 are aligned. First substrate 1 and second substrate 2 are each made of non-pliable base material such as epoxy resin.

On the surfaces (mounting surfaces) of first substrate 1 and second substrate 2, connecting pads to connect electronic components are formed; on the surfaces (mounting surfaces) and inner surfaces of first substrate 1 and second substrate 2, wiring patterns to structure electrical circuits are formed.

On the mounting surfaces of first substrate 1 and second substrate 2, electronic components 7, 8 are placed and connected to connecting pads according to their requirements. Electronic components 7, 8 are connected with each other through connecting pads and wiring patterns.

Wiring board 19 is placed, for example, in the casing of a cell phone device. Under such circumstance, electronic component 7 placed in fewer-layer section 14 is structured, for example, with the keypad of a keyboard; and electronic component 8 placed in multi-layer section 13 is structured with an electronic chip, IC module, functional components and others. Also, in the step portion formed by multi-layer section 13 and fewer-layer section 14, for example, a thin-type battery is placed.

Next, a detailed structure of wiring board 19 having the above overall structure is described in reference to FIG. 2. As illustrated, first substrate 1 and second substrate 2 are laminated by sandwiching base substrate 3 between them. One end (the left end as illustrated in the drawing) of base substrate 3 is made to be flush with first substrate 1 and second substrate 2. Base substrate 3 is made of a highly rigid material such as glass epoxy resin. Base substrate 3 is made 50-100 μm, preferably about 100 μm.

Base substrate 3 is formed to be shorter than second substrate 2, and between first substrate 1 and second substrate 2, groove (hereinafter referred to as “interlayer groove section”) 11 is formed. Interlayer groove section 11 is an aperture. The groove may be filled with elastic material such as silicon gel and silicon oil, or viscous material or others. When wiring board 19 receives an impact from being dropped, the groove aperture or silicon gel or silicon oil that is filled in the interior portion of the groove cushions the impact as a shock-absorbing layer. Therefore, by being structured in this way, tolerance to impact from being dropped may be improved.

First substrate 1 has a structure of laminated multiple insulation layers (1 a, 1 b, 1 c). Each insulation layer is made of epoxy resin or the like with a thickness approximately 10 μm-60 μm. On the upper surface of insulation layer (1 a), between epoxy-resin layers (1 a) and (1 b), between insulation layers (1 b) and (1 c) and on the lower surface of insulation layer (1 c), wiring patterns (111 a, 111 b, 111 c, 111 d) are each formed. Each wiring pattern (111 a, 111 b, 111 c, 111 d) electrically connects required portions inside the circuit substrate.

Second substrate 2 also has a structure of laminated multiple insulation layers (2 a, 2 b, 2 c) made of epoxy resin or the like with a thickness approximately 10 μm-60 μm. On the lower surface of insulation layer (2 a), between epoxy-resin layers (2 a) and (2 b), between insulation layers (2 b) and (2 c) and on the upper surface of insulation layer (2 c), wiring patterns (211 a, 211 b, 211 c, 211 d) are each formed. Each wiring pattern (211 a, 211 b, 211 c, 211 d) electrically connects required portions inside the circuit substrate.

On the exposed portion of the lower surface of first substrate 1 and the exposed portion of the upper surface of the second substrate, adhesion prevention layers 12 as a protective insulation layer are formed. At the step portion created when laminating first substrate 1 and second substrate 2, conductive pattern (111 d) is formed. Also, to the right of conductive pattern (111 d) formed at the step portion, conductive pattern (111 d) is formed as well.

Keypad 7 is placed on the conductive pattern formed on the surface of fewer-layer section 14. Further, using solder 9, electronic chip 8 is anchored and connected to wiring patterns and built-up vias 4 through connecting pads 10. For solder 9, Sn/Ag/Cu was used.

Moreover, through-hole 63 is formed, penetrating base substrate 3, further penetrating first substrate 1 and second substrate 2, and connecting wiring pattern (111 a) of first substrate 1 and wiring pattern (211 d) of second substrate 2. The inner surface of through-hole 63 is plated so as to electrically connect wiring patterns. The area enveloped by plated through-hole 63 may be filled with resin such as epoxy-resin. The term “through-hole” refers to an electrical connection between conductors using a hole or aperture. In general, a through hole may be referred to as a plated through-hole. A through-hole can provide a conductive connection between a conductor formed on one end of the through-hole to a conductor formed on the other end of the through-hole. For example, a through-hole can provide a conductive connection between outer layers of a multilayer printed circuit board and/or may provide conductive connection to or between inner circuits of a multilayer printed circuit board. In forming a through hole, a penetrating hole is first formed by drilling, and a conductor is formed in the penetrating hole by metal plating (such as copper plating). In addition to providing electrical connection as noted above, a through-hole can receive the terminal of an electronic part for insertion mounting and fixing the electronic part to a printed circuit board.

In first substrate 1 and second substrate 2, multiple built-up vias 4 are formed. Built-up vias 4 are structured by stacking vias 44 formed in each insulation layer (1 a-1 c, 2 a-2 c). Built-up vias 4 connect required portions of wiring patterns (111 a-111 d) and also connect required portions of wiring patterns (211 a-211 d). On the inner surface of each via 44 forming built-up via 4, a conductive layer made of plated copper or the like is formed. Thus, the term via, as used herein, means an opening formed in a substrate such as an insulating layer. As shown in FIG. 3, the interior portion of each via 44 is filled with conductor such as copper. However, as shown in FIG. 4, the interior portion of via 44 may be filled with resin such as epoxy-resin.

Wiring board 19 having the above structure, for example, transmits operational signals from keypad 7 to an IC chip through built-up vias 4, wiring patterns (111 a-111 d) and through-hole 63, and the signals are then processed at the IC chip. By doing so, varieties of signal processing may be conducted.

Also, as described above, wiring board 19 is structured with multi-layer section 13 and fewer-layer section 14 and has a step portion. And at the lower portion of fewer-layer section 14, a large-volume component such as a cell-phone battery may be placed.

Base substrate 3 is made of highly rigid material such as glass-epoxy resin. Multi-layer section 13, because of base substrate 3 placed there, is highly rigid compared with fewer-layer section 14. On the other hand, fewer-layer section 14 is relatively flexible compared with multi-layer section 13. Thus, it is possible to place electronic components on either sections 13 or 14 according to the reliability level they require.

Also, for example, when the electronic device is dropped and an impact or the like is exerted on wiring board 19, due to the relative flexibility of fewer-layer section 14 compared with multi-layer section 13, fewer-layer section 14 vibrates as shown by arrow 37 in FIG. 5. Since portions of fewer-layer section 14 vibrate, the impact from being dropped or the like is converted to vibration movement energy, and the impact is absorbed accordingly. As a result, the wiring connecting the electronic components mounted on wiring board 19 may seldom rupture.

Also, built-up via 4 is structured as a stacked via made by laminating multiple vias 44. By making such a stacked interlayer connection structure, the wiring length may be shortened, and thus preferable for mounting electronic components requiring large amount of electricity.

Moreover, built-up via 4 has a certain degree of mobility. Therefore, for example, when the electronic device is dropped and an impact is exerted on wiring board 19, the impact may be absorbed at built-up via 4 through the movement of built-up via 4 as shown by arrows 38, 39 in FIG. 6. As a result, the wiring connecting the electronic components mounted on wiring board 19 may seldom rupture.

In addition, through-hole 63, which penetrates base substrate 3 and further penetrates first substrate 1 and second substrate 2, is formed and the inner surface of through-hole 63 is plated (or filled with resin). Thus, as shown in FIG. 2, if shearing force (Fs) is exerted on the wiring board from a horizontal direction, through-holes 63 may counter the shearing force, thus preventing first substrate 1 and second substrate 2 from sliding off.

Further, if solid material or the like is filled in interlayer groove portion 11, when the impact from being dropped is exerted on the wiring board, interlayer groove portion 11 cushions the impact as a shock-absorbing layer. Accordingly, when interlayer groove portion 11 is formed, by improving tolerance to impact from being dropped, the wiring connecting the electronic components mounted on the wiring board may seldom rupture.

Also, in certain circumstances, two wiring boards of the present invention may be combined and sold in such a way that each fewer-layer section 14 is closely placed to provide compact shipment of the boards as will be further discussed with respect to FIG. 7T below. Here, if a wiring pattern is formed at the step portion created when first substrate 1 and second substrate 2 are laminated, in a circumstance when a user, such as a device manufacturer, uses the combined wiring boards of the present invention separately, warping of the wiring boards may be prevented. Namely, multi-layer section 13, because of base substrate 3 deposited there, is rigid compared with fewer-layer section 14. Thus, when a user separates the combined wiring boards of the present invention, warping does not occur at multi-layer section 13. On the other hand, fewer-layer section 14 is flexible compared with multi-layer section 13. Thus, when a user separates the combined wiring boards of the present invention, warping could possibly occur at fewer-layer section 14, especially at the step portion of fewer-layer section 14 created when first substrate 1 and second substrate 2 are laminated. However, if a wiring pattern is formed at the step portion, in a circumstance when a user or the like uses the combined wiring boards of the present invention separately, warping may be prevented.

In the following, a method of manufacturing wiring board 19 according to the present invention is described.

First, as shown in FIG. 7A, dummy core 52 which later forms adhesion prevention layer 12 is prepared. Dummy core 52 is, for example, formed with a C-stage epoxy-resin. On dummy core 52, copper foil 51 is deposited.

Next, as shown in FIG. 7B, by patterning copper foil 51, conductive pattern (111 d) is formed at a predetermined position.

Then, as shown by arrows in FIG. 7C, dummy core 52 is cut by a laser or the like (represented by the arrows in FIG. 7C) to adjust its length to a length preferred for use in wiring board 19. As seen in FIG. 7C, the dummy core 52 is cut into dummy cores 52 a and 52 b, which will be used to form separate wiring boards as described below.

In addition, as shown in FIG. 7D, core 55, which later functions as base substrate 3, is prepared. Core 55 is made, for example, of highly rigid material such as glass-epoxy resin. On both surfaces of core 55, copper foil 54 is deposited.

Next, as shown in FIG. 7E, by patterning copper foil 54, conductive patterns (111 d, 211 a) are formed to structure wiring patterns.

Next, as shown by an arrow in FIG. 7F, in core 55 using a laser or the like, a hole to insert dummy core 52 is formed.

Next, as shown in FIG. 7G, cut-out dummy cores (52 a, 52 b) are placed in such a way that conductive pattern (111 d) and conductive pattern (211 a) are laminated facing inward. Then, laminated dummy cores (52 a, 52 b) and cut core 55 are horizontally connected. In the following, on the top and bottom of dummy cores (52 a, 52 b) and core 55, prepreg (62 a, 62 b) are laminated. For prepreg (62 a, 62 b), low-flow prepreg impregnated with low-flow epoxy-resin is preferred. Then, on the surfaces of prepreg (62 a, 62 b), copper foils (61 a, 61 b) are deposited.

Next, as shown in FIG. 7H, the laminated layers shown in FIG. 7G are pressure-pressed, as schematically represented by arrows in FIG. 7H. Pressure pressing is, for example, conducted by hydraulic power using hydraulic pressing equipment under conditions calling for temperature of 200° C., pressure of 40 kgf and pressing time of three (3) hours. By doing so, resin leaks from the prepreg, and the prepreg (62 a, 62 b) and core material 55 will be integrated accordingly. At this time, since dummy cores (52 a, 52 b) is made of a C-stage epoxy-resin, the materials in dummy core 52 are not integrated with each other. For pressure pressing, vacuum pressing may be employed instead of hydraulic pressing. By conducting vacuum pressing, bubbles may be kept from mixing into the resin which structures the insulation layers. Vacuum pressing is conducted, for example, for an hour. Peak heating temperature is set, for example, at 175° C.; and vacuum-pressing pressure is set, for example, at 3.90×10⁶ [Pa].

Next, as shown in FIG. 7I, by removing the unnecessary portions of laminated copper foil 61 shown in FIG. 7H, wiring patterns 62 c are formed.

Next, as shown in FIG. 7J, epoxy resin (72 a, 72 b) is further laminated to form inner layers. On both surfaces of epoxy resin (71 a, 71 b), copper foil 71 is formed. Then, pressure pressing is conducted, as shown by the arrows in FIG. 7J. Pressure pressing may be conducted, for example, by hydraulic power using hydraulic pressing equipment, or by vacuum pressing.

Then, as shown in FIG. 7K, vias 44 are formed. Namely, in epoxy resin 72 made of insulation resin, via-hole openings are formed. Those openings may be formed by a laser beam. Then, to remove resin residue remaining on the side and bottom surfaces of the openings formed by the laser, a desmear treatment is preferably performed. The desmear treatment is performed using an oxygen plasma discharge treatment, a corona discharge treatment, an ultra-violet laser treatment or an exima laser treatment. In the openings formed by beaming a laser, conductive material is filled to form filled via holes. As for conductive material, conductive paste or metal plating formed by an electrolytic plating process is preferred. For example, vias 44 are filled with conductor such as copper plating. To reduce the manufacturing cost and improve productivity by simplifying the filled-via forming step, filling with a conductive paste is preferred. For example, a conductive paste (such as thermo-set resin containing conductive particles) may be printed by screen-printing, filled in vias 44 and set. By filling the interior portion of vias 44 with the same conductive paste material, connection reliability when thermo-stress is exerted on vias 44 may be improved. On the other hand, regarding connection reliability, metal plating formed by an electrolytic plating process is preferred. Especially, electrolytic copper plating is preferred.

Then, as shown in FIG. 7L, by removing the unnecessary portions of copper foil 71, inner-layer patterns 71 c are formed.

Next, as shown in FIG. 7M, after inner layers and vias are further formed, epoxy-resin (81 a, 81 b) is laminated to form outer layers. On surfaces of epoxy-resin (81 a, 81 b), copper foil (82 a, 82 b) is deposited. Here, a copper foil sheet with resin (Resin Copper Film: RCF) may be deposited and pressed.

Next, as shown in FIG. 7N, vias 64 are formed in the RCF. Further, in the laminated layers shown in FIG. 7M, holes 63 are bored by a drill. The holes 63 penetrate the base substrate and insulation layers (62 a, 62 b) formed on both sides of the base substrate. By doing so, through-holes 63 are formed. In addition, using copper plating or the like, the interior portions of the vias 64 and through-holes 63 are provided with a conductor. As seen in FIG. 7O, to requirements, by patterning the surface copper foil, conductive patterns are formed.

Next, as shown in FIG. 7O, the interior portions of through-holes 63 are filled with epoxy resin, and by removing unnecessary portions of copper foils (82 a, 82 b), outer-layer patterns 82 c are formed.

Next, as shown in FIG. 7P, solder resists 83 are formed. Here, the solder resist indicates heat-resistant coating material, which is used when applying solder, to cover the portions which are intended to keep the solder from adhering thereto. For solder-resist varieties, photo-setting type solder resist and thermo-setting type solder resist may be used. For a coating method, a screen-printing method or curtain-coating method may be used.

Next, as shown in FIG. 7Q, to protect outer-layer patterns, gold plating 91 is performed by chemical plating. Other than chemical plating, methods such as fusion plating and electrical plating may be used. Moreover, methods other than gold plating, such as alloy plating, may be used.

Next, as shown by arrows 40 in FIG. 7R, laser beams from laser processing equipment, for example a CO₂ laser, are irradiated using conductive patterns (111 d) as a stopper to cut insulation layers, and the copper foil sheet with resin (RCF). Here, the thickness of conductive patterns (111 d) is preferred to be made approximately 5-10 μm: if too thin, laser beams penetrate the pattern; and if too thick, conductive patterns with a fine line width are difficult to form.

By laser cutting as shown in FIG. 7R, interlayer groove portions (11 a, 11 b) are also formed. Namely, by laser cutting, using adhesion prevention layer 12 formed in first substrate 1 and adhesion prevention layer (12 a, 12 b) formed in second substrate 2 as groove side-walls, and one surface of base substrate 3 as the groove bottom, interlayer groove portions (11 a, 11 b) are formed.

Lastly, as shown in FIG. 7S, electronic components 92 are mounted. Electronic components 92 are electronic chip 8 and keypad 7. Further, interlayer groove portions (11 a, 11 b) may be filled with elastic material, viscous material or the like as depicted by the darkened portion shown in the interlayer groove portions in FIG. 7S.

Further, as shown in FIG. 7T, wiring board (19A) and wiring board (19B) are used separately. In such a circumstance, since adhesion prevention layers (12 a, 12 b) are formed, wiring board (19A) and wiring board (19B) may be separated by a simple process to be used separately. Regarding a wiring board according to the present invention, when an electronic device such as a cell phone receives an impact from being dropped or the like, connection breakage of electronic components or the like mounted in the wiring board may be prevented. Also, when being shipped to a user, the wiring board can be handled compactly, and when being used by the user, the combined wiring boards can be separated easily.

In the First Embodiment, to interconnect wiring patterns in first substrate 1 and second substrate 2, through-hole 63 is formed. However, the present invention is not limited to such embodiment. For example, an Interstitial Via Hole (“IVH”) may be formed to connect conductive layers. As shown in FIG. 8, an IVH 64 is formed between first substrate 1 and second substrate 2. The term IVH means a conductive structure which is formed by a hole that penetrates a base substrate or insulating layers, for example, but does not penetrate through a multilayered printed board itself, and plating the hole to electrically connect two or more conductive layers. An IVH occupies a space required for a connection, and includes, for example, a blind via hole structure formed in an outer layer of a multilayered printed wiring board and a buried via hole structure formed in an inner layer of the multilayered printed wiring board. Through the IVH 64 shown in the example of FIG. 8, wiring patterns in first substrate 1 and second substrate 2 are connected and shearing force may be countered as well.

If a wiring board has IVH 64, as shown in FIG. 8, when the wiring board receives shearing force (Fs) from a horizontal direction, the wiring board can counter shearing force (Fs) through an anchoring effect. Accordingly, when an electronic device such as a cell phone receives an impact from being dropped or the like, the wiring connecting electronic components mounted in the wiring board may seldom rupture.

The method of manufacturing a wiring board according to the Second Embodiment is the same as the method of manufacturing a wiring board according to the First Embodiment in reference to FIGS. 7A-7H. However, FIGS. 9A-9E show how the method of the second embodiment deviates from FIG. 7. As seen in FIG. 9A, in the laminated layers shown in FIG. 7H, holes are bored by a drill. Those holes later become IVHs 64. Further, copper foil 61 is provided.

Next, as shown in FIG. 9B, by removing the unnecessary portions of copper foil 61, inner-layer patterns 61 c are formed.

As shown in FIG. 9C, epoxy resin (72 a, 72 b) is further laminated to form inner layers. On both surfaces of epoxy resin (72 a, 72 b), copper foil (71 a, 71 b) is deposited.

Next, as shown in FIG. 9D, after the lamination in FIG. 9C, pressure pressing is conducted as shown by the arrows in FIG. 9D. Pressure pressing may be conducted, for example, by hydraulic power using hydraulic pressing equipment, or by vacuum pressing. Portions of epoxy resin 72 (shown by dark areas) are filled in the interior portion of the holes which later become IVHs 64.

Then, as shown in FIG. 9E, by forming vias 44 in a stacked configuration at both ends of the formed holes, IVHs 64 are formed. After that, as shown in FIG. 7M, vias are further stacked over the formed vias. By plating IVHs 64, interlayer connection is carried out. In the area enveloped by the plated portions, resin such as epoxy resin is filled.

As shown in FIG. 10, in the Third Embodiment, at the portion where adhesion prevention layer 12 is made flush with the edge of second substrate 2, opening 5 is formed. The rest of the structure is the same as in the First Embodiment. Under opening 5, part of wiring pattern (111 d) is positioned. Inside the groove which is structured with opening 5 and wiring pattern (111 d) placed underneath is an aperture. The groove may be filled with elastic material such as silicon gel or silicon oil or viscous material. When wiring board 19 receives an impact from being dropped, the aperture inside the groove, or silicon gel or silicon oil filled in the groove, cushions the impact as a shock-absorbing layer. Therefore, by making such a structure, tolerance to impact from being dropped may be improved.

Also, if solid material or the like is filled in opening 5, the filled solid material or the like may play a role in decreasing warping at the juncture of multi-layer section 13 and fewer-layer section 14 where the number of layers is reduced. Accordingly, at the juncture of multi-layer section 13 and fewer-layer section 14, cracks may be prevented. Furthermore, if opening 5 is filled, for example, with solid material such as resin, the filled solid material plays a role in protecting conductive pattern (111 d) mounted on first substrate 1. Therefore, tolerance to corrosion on conductive pattern (111 d) may be improved.

A method of manufacturing a wiring board according to the Third Embodiment is the same as the method of manufacturing a wiring board according to the First Embodiment in reference to FIGS. 7A-7F, FIGS. 7H-7T. However, instead of FIG. 7G, as shown in FIG. 11A, cut-out dummy cores (52 a, 52 b) are placed in a way so that conductive patterns (111 d) are laminated facing outward. Moreover, instead of FIG. 7S, as shown in FIG. 11B, opening 5 is filled with viscous material such as silicon oil.

In the First Embodiment, base substrate 3 was made of glass-epoxy resin. However, as shown in FIGS. 12 and 13, in the Fourth Embodiment, base substrate 3 is made containing base material of resin-impregnated inorganic fiber. By being structured as such, since base substrate 3 contains base material of resin-impregnated inorganic fiber, tolerance to warping may be improved.

The base material made of resin-impregnated inorganic fiber is formed by setting a prepreg. Prepreg is made by impregnating glass-cloth of inorganic fiber with epoxy-resin, then preliminarily thermosetting the resin to advance the level of setting. Although the resin used to form prepreg is preferred to have low-flow characteristics, those having regular flow characteristics may be used as well. Also, the prepreg may be formed by reducing the amount of epoxy-resin impregnated in the glass-cloth of inorganic fiber.

As for the inorganic fiber, it is not limited to glass-cloth, but may include, for example, alumina fiber, carbon fiber (carbon fiber), silicon carbide fiber or silicon nitride fiber.

In the method of manufacturing a wiring board according to the Fourth Embodiment, referring to FIG. 7D, as the material to form core 55, a base material of resin-impregnated inorganic fiber is used. The rest is the same as the method of manufacturing a wiring board according to the First Embodiment.

In the above-described First Embodiment, base substrate 3 was made of glass-epoxy resin. First substrate 1 and second substrate 2 were made of epoxy resin. However, the combination of material for base substrate 3 and material for first substrate 1 and second substrate 2 is not limited to the above embodiment. As shown in FIGS. 14-15, in the Fifth Embodiment, base substrate 3 is made containing base material of resin-impregnated inorganic fiber; and first substrate 1 and second substrate 2 are made containing inorganic filler composite resin. By such structuring, since base substrate 3 includes base material containing resin-impregnated inorganic fiber, tolerance to warping may be improved. Accordingly, when an electronic device such as a cell phone receives an impact from being dropped or the like, the wiring connecting electronic components mounted in the wiring board may seldom rupture.

Inorganic filler composite resin may be made by combining silica filler or glass filler with epoxy resin. In addition to epoxy resin, or other than epoxy resin, polyimide, polycarbonate, polybutylene-telephtarate or polyacrylate may be used.

For silica filler, fused silica (SiO₂) or crystalline silica (SiO₂) may be used. Also, for glass filler, aluminum oxide (Al₂O₃), magnesium oxide (MgO), or boron nitride (BN), aluminum nitride (AlN) may be used. Furthermore, for inorganic filler, it is not limited to silica filler or glass filler, but antimony trioxide, antimony pentaxide or magnesium hydroxide may be used.

In the method of manufacturing a wiring board according to the Fifth Embodiment, referring to FIG. 7D, as the material to form core 55, base material of resin-impregnated inorganic fiber is used. In addition, for the resin to be laminated in reference to FIGS. 7G, 7J and 7M, inorganic filler composite resin is used. The rest is the same as the method of manufacturing a wiring board according to the First Embodiment.

In the above-described First Embodiment, base substrate 3 was made of glass-epoxy resin. And first substrate 1 and second substrate 2 were made of epoxy resin. However, the combination of material for base substrate 3 and material for first substrate 1 and second substrate 2 is not limited to the above embodiment. As shown in FIGS. 16-17, in the Sixth Embodiment, base substrate 3 is made containing inorganic filler composite resin; and first substrate 1 and second substrate 2 are made containing resin-impregnated inorganic fiber. By structuring so, since at least either first substrate 1 or second substrate 2 is reinforced with inorganic fiber, tolerance to warping may be improved. Accordingly, when an electronic device such as a cell phone receives an impact from being dropped or the like, the wiring connecting electronic components mounted in the wiring board may seldom rupture.

The above-described inorganic material such as inorganic fiber or inorganic filler has small thermo-expansion rates and low coefficient of elasticity compared with resin of organic material. Therefore, when inorganic material such as inorganic fiber or inorganic filler is combined, alignment gaps between connecting lands may be reduced.

In the method of manufacturing a wiring board according to the Sixth Embodiment, referring to FIG. 7D, as the material to form core 55, inorganic filler composite resin is used. In addition, for the resin to be laminated in reference to FIGS. 7G, 7J and 7M, a base material of resin-impregnated inorganic fiber is used. The rest is the same as the method of manufacturing a wiring board according to the First Embodiment.

In the Second Embodiment, between first substrate 1 and second substrate 2, IVH 64 was formed. In the Second Embodiment, on the top and bottom ends of IVH 64, vias 44 were formed. However, the configuration of IVH 64 is not limited to the above embodiment. As shown in FIG. 18, IVH 64 which does not have vias on the top and bottom ends may be formed.

In a wiring board having IVH 64 which does not have vias on the top and bottom ends, when shearing force is exerted on the wiring board from a horizontal direction, the shearing force may be countered through an anchoring effect. Accordingly, when an electronic device such as a cell phone receives an impact from being dropped or the like, the wiring connecting electronic components mounted on the wiring board may seldom rupture.

Here, in the present invention, IVHs 64 include those not having vias on top and bottom ends, as well as those having such vias.

The method of manufacturing a wiring board according to the Second Embodiment is the same as the method of manufacturing a wiring board according to the First Embodiment in reference to FIGS. 7A-7H, and after that, also the same as the method of manufacturing a wiring board according to the Second Embodiment in reference to FIGS. 9A-9D. Then, as shown in FIG. 19A, vias 44 are formed at the portions excluding both ends of the holes formed in reference to FIG. 9D.

Then, as shown in FIG. 19B, by removing the unnecessary portions of copper foil 71, inner-layer patterns 71 c are formed. Furthermore, as shown in FIG. 19C, after further forming inner layers and vias, epoxy resin 81 is laminated to form outer layers. On both surfaces of epoxy resin 81, copper foil 82 is deposited. After that, as shown in FIG. 7P, solder resists 83 are formed and electronic component 92 is mounted as shown in FIG. 7S.

In a wiring board according to the First Embodiment of the present invention, first substrate 1 and second substrate 2 are in a stratum structure having a rectangular outline. However, they are not limited to the above structure, but may be in a stratum structure having a circular, hexagonal or octagonal outline.

Also, in the First Embodiment, first substrate 1 and second substrate 2 are made of epoxy resin. However, first substrate 1 and second substrate 2 are not limited to such, but may be made of polyimide, polycarbonate, polybutylene-telephtarate or polyacrylate. In addition, if first substrate 1 and second substrate 2 are made of epoxy resin, naphthalene-type epoxy resin, dicyclo-penta-diene-type epoxy resin, biphenyle-type epoxy resin or bisphenole-type epoxy resin may be used.

In the First Embodiment, as solder 9, Sn/Ag/Cu was used. However, solder 9 is not limited to such; solder containing antimony, tin, lead, indium or copper may be used. Also, eutectic crystal metals such as Sn/Sb, Sn/Ag, Sn/Pb or Sb/Cu may be used as well. Among such eutectic crystal metals, to avoid having a bad influence on the substrates, using metals having relatively low melting temperatures, 250° C. or lower, is preferred.

In the First Embodiment, interlayer groove portion 11 is filled with silicon gel of viscous silicon. However, filling interlayer groove portion 11 is not limited to such, but solid material may be used. As for the solid material to be filled in interlayer groove portion 11, high-polymer rubber is preferred as a solid material having viscosity and elasticity. Specifically, butyl-rubber, isoprene rubber, butadiene rubber, styrene-butadiene rubber or ethylene-propylene rubber may be used. Moreover, interlayer groove portion 11 may be filled with a gas. As the gas to be filled in interlayer groove portion 11, a rare gas such as argon, or nitrogen or oxygen may also be used.

In the Second Embodiment, in opening 5, silicon gel of viscous silicon is filled. However, the material to be filled in opening 5 is not limited to such, but solid material may be used. As solid material to be filled in opening 5, high-polymer rubber as a solid material having viscosity and elasticity is preferred. Specifically, butyl-rubber, isoprene rubber, butadiene rubber, styrene-butadiene rubber or ethylene-propylene rubber may be used. As the material to be filled in opening 5, a liquid or solid material is preferred, but a gas may be used. In such a case, as the gas to be filled in opening 5, a rare gas such as argon, or nitrogen or oxygen may be used.

In addition, first substrate 1 may not need to be formed single-layered, but may be formed multi-layered. Namely, first substrate 1 may be structured with a lower-layer insulation layer and an upper-layer insulation layer. Here, a lower-layer insulation layer indicates the insulation layer formed close to base substrate 3; and an upper-layer insulation layer indicates an insulation layer formed on the outer surface of the wiring board. Furthermore, first substrate 1 may be structured with a lower-layer insulation layer, an upper-layer insulation layer and an intermediate insulation layer placed in between. The intermediate insulation layer may be made multi-layered. In the First Embodiment, the lower-layer insulation layer corresponds to epoxy-resin layer (1 c), the intermediate insulation layer corresponds to epoxy-resin layer (1 b) and the upper-layer insulation layer corresponds to epoxy-resin layer (1 a).

Also, the second substrate may not need to be formed single-layered, but may be formed multi-layered. Second substrate 2 may also be structured with a lower-layer insulation layer and an upper-layer insulation layer. Furthermore, second substrate 2 may be structured with a lower-layer insulation layer, an upper-layer insulation layer and an intermediate insulation layer placed in between. In the First Embodiment, the lower-layer insulation layer corresponds to epoxy-resin layer (2 a), the intermediate insulation layer corresponds to epoxy-resin layer (2 b) and the upper-layer insulation layer corresponds to epoxy-resin layer (2 c). On top of the upper-layer insulation layer and lower-layer insulation layer, conductive patterns are formed. And, those conductive patterns may be connected with each other through vias 44.

The present invention may be employed in a wiring board which can mount electronic components, specifically, in a wiring board which can mount electronic components for compact electronic devices.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. 

What is claimed is:
 1. A wiring board comprising: a first substrate; a second substrate having a smaller mounting area than a mounting area of the first substrate; a base substrate laminated between the first substrate and the second substrate such that the first substrate extends beyond an edge of the second substrate and the edge of the second substrate forms a step portion with respect to the first substrate; an Interstitial Via Hole structure penetrating through the base substrate; and a plurality of vias formed in at least one of the first substrate and the second substrate, wherein the first substrate and the second substrate are laminated to the base substrate such that the first substrate and the second substrate form a first end portion of the wiring board, the first substrate forms a second end portion of the wiring board on an opposite side of the first end portion of the wiring board, the second substrate does not form the second end portion of the wiring board, and the base substrate has an interlayer groove portion formed at the edge of the second substrate between the first substrate and the second substrate.
 2. The wiring board according to claim 1, wherein the interlayer groove portion is filled with at least one of a gas, a liquid and a solid material.
 3. The wiring board according to claim 1, further comprising a warping prevention portion formed at the step portion.
 4. The wiring board according to claim 1, wherein: the base substrate comprises a resin-impregnated inorganic fiber; the first substrate comprises at least one of an inorganic filler composite resin and a pliable resin; and the second substrate comprises at least one of an inorganic filler composite resin and a pliable resin.
 5. The wiring board according to claim 4, wherein the resin-impregnated inorganic fiber comprises a glass cloth.
 6. The wiring board according to claim 4, wherein at least one of the first substrate and the second substrate comprises the inorganic filler which comprises at least one of a silica filler and a glass filler.
 7. The wiring board according to claim 1, wherein: the base substrate comprises an inorganic filler composite resin; and at least one of the first substrate and the second substrate comprises a base material of a resin-impregnated inorganic fiber.
 8. The wiring board according to claim 7, wherein the inorganic fiber comprises a glass cloth.
 9. The wiring board according to claim 7, wherein the inorganic filler comprises at least one of a silica filler and a glass filler.
 10. The wiring board according to claim 1, further comprising: a conductive pattern formed on the first substrate; and a conductive pattern formed on the second substrate, wherein the conductive pattern on the first substrate and the conductive pattern on the second substrate are electrically connected by the Interstitial Via Hole structure.
 11. The wiring board according to claim 1, further comprising a conductive layer formed on an inner surface of at least one of said vias by plating, wherein the at least one of the vias is filled with a metal.
 12. The wiring board according to claim 1, further comprising a conductive layer formed on an inner surface of at least one of said vias by plating, wherein the at least one of the vias is filled with a resin.
 13. The wiring board according to claim 1, further comprising a plurality of stacked vias provided on each end of the Interstitial Via Hole structure, respectively.
 14. A wiring board comprising: a first substrate including a first lower-layer insulation layer and a first upper-layer insulation layer; a second substrate having a smaller mounting area than a mounting area of the first substrate and including a second lower-layer insulation layer and a second upper-layer insulation layer; a base substrate laminated between the first substrate and the second substrate such that the first substrate extends beyond an edge of the second substrate and the edge of the second substrate forms a step portion with respect to the first substrate; an Interstitial Via Hole structure penetrating through the base substrate; a plurality of vias formed in at least one of the first substrate and the second substrate; a plurality of conductive patterns formed on each of the first and second upper-layer insulation layers; and a plurality of conductive patterns formed on each of the first and second lower-layer insulation layers, wherein the first substrate and the second substrate are laminated to the base substrate such that the first substrate and the second substrate form a first end portion of the wiring board, the first substrate forms a second end portion of the wiring board on an opposite side of the first end portion of the wiring board, the second substrate does not form the second end portion of the wiring board, and the conductive patterns on the first and second upper-layer insulation layers are each connected to a respective conductive pattern of the conductive patterns on the first and second lower-layer insulation layers.
 15. A wiring board comprising: a first substrate; a second substrate having a smaller mounting area than a mounting area of the first substrate; a base substrate laminated between the first substrate and the second substrate such that the first substrate extends beyond an edge of the second substrate and the edge of the second substrate forms a step portion with respect to the first substrate; a through hole structure penetrating through the base substrate; and a plurality of vias formed in at least one of the first substrate and the second substrate, wherein the first substrate and the second substrate are laminated to the base substrate such that the first substrate and the second substrate form a first end portion of the wiring board, the first substrate forms a second end portion of the wiring board on an opposite side of the first end portion of the wiring board, the second substrate does not form the second end portion of the wiring board, and the base substrate has an interlayer groove portion formed at the edge of the second substrate between the first substrate and the second substrate.
 16. The wiring board according to claim 15, wherein the interlayer groove portion is filled with at least one of a gas, a liquid and a solid material.
 17. The wiring board according to claim 15, further comprising a warping prevention portion formed at the step portion.
 18. The wiring board according to claim 15, wherein: the base substrate comprises resin-impregnated inorganic fiber; the first substrate comprises at least one of an inorganic filler composite resin and a pliable resin; and the second substrate comprises at least one of an inorganic filler composite resin and a pliable resin.
 19. The wiring board according to claim 18, wherein the resin-impregnated inorganic fiber comprises a glass cloth.
 20. The wiring board according to claim 18, wherein at least one of the first substrate and the second substrate comprises the inorganic filler which comprises at least one of a silica filler and a glass filler.
 21. The wiring board according to claim 15, wherein: the base substrate comprises an inorganic filler composite resin; and at least one of the first substrate and the second substrate comprises a base material of a resin-impregnated inorganic fiber.
 22. The wiring board according to claim 21, wherein the inorganic fiber comprises a glass cloth.
 23. The wiring board according to claim 21, wherein the inorganic filler comprises at least one of a silica filler and a glass filler.
 24. The wiring board according to claim 15, further comprising: a conductive pattern formed on the first substrate; and a conductive pattern formed on the second substrate, wherein the conductive pattern on the first substrate and the conductive pattern on the second substrate are electrically connected by the through hole structure.
 25. The wiring board according to claim 15, further comprising a conductive layer formed on an inner surface of at least one of said vias by plating, wherein the at least one of the vias is filled with a metal.
 26. The wiring board according to claim 15, further comprising a conductive layer formed on an inner surface of at least one of said vias by plating, wherein the at least one of the vias is filled with a resin.
 27. The wiring board according to claim 15, further comprising a plurality of stacked vias provided on each end of the Interstitial Via Hole structure, respectively.
 28. A wiring board comprising: a first substrate including a first lower-layer insulation layer and a first upper-layer insulation layer; a second substrate having a smaller mounting area than a mounting area of the first substrate and including a second lower-layer insulation layer and a second upper-layer insulation layer; a base substrate laminated between the first substrate and the second substrate such that the first substrate extends beyond an edge of the second substrate and the edge of the second substrate forms a step portion with respect to the first substrate; a through hole structure penetrating through the base substrate; a plurality of vias formed in at least one of the first substrate and the second substrate; a plurality of conductive patterns formed on each of the first and second upper-layer insulation layers; and a plurality of conductive patterns formed on each of the first and second lower-layer insulation layers, wherein the first substrate and the second substrate are laminated to the base substrate such that the first substrate and the second substrate form a first end portion of the wiring board, the first substrate forms a second end portion of the wiring board on an opposite side of the first end portion of the wiring board, the second substrate does not form the second end portion of the wiring board, and the conductive patterns on the first and second upper-layer insulation layers are each connected to a respective conductive pattern of the conductive patterns on the first and second lower-layer insulation layers. 